Which of the following is correct about succession? cause of environment change during succession:-

$\begin{array}{1 1}(a)\;\text{Changes in environment bad to change in species } \\ (b)\;\text{Natural forces change the environment from time to time} \\(c)\;\text{Pre-existing species change make the environment unsuitable for their survival} \\(d)\;\text{Pre-existing species change make the environment suitable for their survival}\end{array}$